Systems and methods for remotely monitoring the exhaust gas composition of motor vehicles are known. For example, a Remote Sensing System (RSS) positioned at a predetermined location along a roadway measures tailpipe emissions as vehicles pass through infrared (IR) and ultraviolet (UV) beams cast across the roadway. IR/UV light is typically reflected back across the roadway (e.g., via a transfer mirror module) to a series of detectors that monitor light intensity at characteristic wavelengths. By measuring the absorption of IR/UV light by the various pollutants in the air, the RSS is able to calculate pollutant concentrations in the exhaust plumes of passing vehicles.
More particularly, RSS exhaust emissions measurements comprise a series of periodic readings spanning a predetermined time period (e.g., approximately one-half second after the passing of a vehicle). Each reading gives the quantity of each gas of interest (e.g., in an exhaust plume) in the beam path. Because the amount of dilution of an exhaust plume at each instant is not known, the individual readings for each gas are ratioed, for example, to a corresponding carbon dioxide (CO2) reading. These ratios of gas amounts remain constant throughout an exhaust plume, and are independent of dilution. The ratios themselves may be useful. For example, the ratios may be directly converted to mass of pollutant per mass of fuel, or can be used in conjunction with the combustion equation to convert the ratios into the gas concentrations which would be measured by a tailpipe probe when corrected for excess air and water. A combustion equation that assumes a particular fuel is used to convert the ratios into gas concentrations.
Remote emissions measurements rely on the ability of each gas of interest to absorb light of only certain wavelengths. A channel (for each gas of interest) compares the amount of light traversing the beam path in its particular frequency before the vehicle with the amount of light after the vehicle, and calculates the amount of gas from the absorbed light. To correct for fluctuations in the light source or light that may be blocked by particles, a reference channel is also used at a frequency where there is no gaseous absorption.
Over time, steady advances in the sophistication and robustness of remote sensing technology, together with the analysis of acquired vehicle emissions data, have resulted in a number of important findings. As an example, it has been determined that the presence of water droplets in vehicle exhaust emission plumes may cause erroneously high hydrocarbon (HC) readings. This may be especially problematic when attempting to measure evaporative emissions (e.g., vapors that vent into the air from hot engines and fuel systems) by noting high HC readings that are uncorrelated with accompanying CO2 readings.
One prior approach for recognizing and rejecting erroneously high HC readings in remote emissions measurements includes the use of an additional IR channel that is twice as sensitive to water droplets as it is to gaseous HC. See WILLIAMS, Mitchell Jared, “Advances in On-Road Remote Sensing: Feat 5000 The Next Generation”, Thesis, University of Denver, MAS 2003 No. 27, August 2003, which is hereby incorporated herein by reference in its entirety. Detections of elevated HC in the HC channel accompanied by readings that are twice as elevated in the additional IR channel are flagged as water droplet “steam” plumes. Use of this hardware-based approach, however, may result in both an increased time and expense associated with hardware configuration and/or calibration.
These and other drawbacks exist.